My setup is this: 3 Devices (1 Firewire, 2 SCSI), the Firewire device has only one volume, and each SCSI device has 3 volumes each.

I would like to unmount one volume from each SCSI device at login. I will create a Platypus App to execute a simple SH script. Then I will add this App to my Login Items in the Accounts PrefPane in System Preferences.

Here is Platypus' link:

http://www.macupdate.com/info.php/id/12046

Download and install Platypus (its free).

Launch Platypus.

Input your App Name (I used UnmountVolumesLogIn), then click on "New" underneath the script path area. Paste the following text (change MyVolume1):

###########

#!/bin/sh
diskutil unmount /Volumes/MyVolume1
exit

###########

Then click "Save"

Then click "Create" and save the App to wherever you desire.

Then add your new app to the login items in the accounts prefpane, and restart.

You many need to add this line to your sh script:

sleep 1

And you also may need to be an administrator user to unmount volumes.